This invention relates generally to a method for using liquefied natural gas as a source of refrigeration in a process for separating a hydrocarbon feed gas mixture to produce a separated ethylene product.
Liquefaction is commonly employed to facilitate transportation of natural gas where great distances separate the source and distribution points. Liquefaction offers the advantage of greatly reducing the volume of the natural gas, e.g., by a factor of roughly 600 at a pressure of one atmosphere. Removal of substantial amounts of sensible and latent heat is required for liquefaction, however; at one atmosphere, for example, natural gas liquefies at approximately -160.degree. C. There is thus considerable "cold" stored in the liquefied natural gas (LNG). Since the ultimate use of natural gas is characteristically at ambient or near ambient temperatures, the LNG is generally revaporized at the point of distribution. With such vaporization, there is opportunity to employ the refrigeration associated with the "cold" and thereby recover work initially expended on the natural gas to liquefy it.
There have been many attempts by the prior art to use the refrigeration associated with LNG by integrating LNG vaporization with a variety of commercial processes, for example, air separation, hydrogen recovery from refinery tail gases, liquefaction of trans-shipment "cold carriers", and olefins (ethylene) production. In particular, much work has been done on integrating LNG revaporization with ethylene plants, since the latter characteristically employ in-plant refrigeration at temperature levels from 0.degree. C. down to -100.degree. C. and lower.
In conventional ethylene plants, the low temperature fractionation steps for separating the various components of the olefin-containing hydrocarbon feed stream are typically conducted at relatively high pressures of 400-600 psig, with refrigeration for the separation steps being supplied by propylene at temperature levels down to about -40.degree. C. and by ethylene at levels down to about -100.degree. C. This refrigeration system, although efficient, is expensive due to the provision of large and costly refrigeration compressors.
In some ethylene plants, methane refrigeration has been employed in conjunction with propylene and ethylene refrigerants in cascaded refrigeration systems. Such cascaded systems permit temperatures as low as -130.degree. C. to be achieved and allow the operating pressures of the fractionation steps to be substantially reduced, for example to 150-200 psig, by virtue of the lower temperatures. Triple-cascaded systems, however, require additional methane refrigeration compressors, and the savings in feed stream compression is not large enough to provide an overall practical advantage for the cascaded system. As a result, methane refrigeration has not been used widely in commercial ethylene plants.
It has been recognized, however, that the process gains which result from the use of methane refrigeration in ethylene production can be realized without penalty by the use of liquefied natural gas (LNG) as a refrigerant, in locations where natural gas distribution networks or use facilities exist in close proximity to the ethylene plant. An efficient utilization of LNG allows the in-plant (propylene-ethylene) refrigeration system to be eliminated or, at the very least, substantially reduced, with corresponding elimination or reduction of the investment and operating costs for the refrigeration compressors. Additionally, the lower temperatures characteristic of LNG permit a reduction in the operating pressures of the fractionation equipment, so that the investment and operating costs for the feed gas compression system are likewise reduced.
Regarding the fractionation steps of the ethylene plant in greater detail, two general process equipment sequences are widely employed. One sequence incoporates a demethanizer column at the head end of the fractionation section followed by de-ethanizer and C.sub.2 splitter columns, depropanizer and C.sub.3 splitter columns, debutanizer column, depentanizer column, and other separation equipment as required. This is perhaps the most generally employed arrangement; however, it has been established that significant process gains are realized in many cases by positioning the depropanizer at the head of the fractionation section, so that C.sub.3 's and lighter are separated from C.sub.4 's and heavier initially. The so-called front-end depropanizer scheme permits better maintenance of olefin (ethylene) purity specifications, and substantially reduces capital, power, and operating costs over a front end demethanizer arrangement. By removing the C.sub.4 's and heavier initially, the subsequent separation equipment can operate at extremely low temperatures without the problems arising from freezing of heavy hydrocarbon components. Operation at such extremely low temperatures allows particularly efficient light component separations. In one especially preferred arrangement for a front-end depropanizer system, the light components in the depropanizer overhead stream are separated and removed in a forecooling recovery section, with the remaining olefin-bearing streams passing to the final separation section. This arrangement, with the forecooling recovery section operating at low temperature, is particularly suitable in connection with the use of LNG as a source of refrigeration.
The efficiency of LNG-supplied refrigeration in any ethylene plant is related to how closely the LNG enthalpy/temperature warming curve is able to approach the corresponding cooling curve for the olefins plant. The "match" between these two curves will determine how well the LNG provides the refrigeration requirements for the plant. There are, however, certain practical considerations which apply to this match. For example, it is desirable to avoid temperature "pinches" (excessively small .DELTA.T's) in the process heat exchangers between the cooling and warming streams. Such pinches require prohibitively large amounts of heat transfer area to achieve the desired heat transfer. In addition, it is necessary to avoid very large temperature differences, since energy losses in heat exchangers are dependent on the temperature differences of the heat exchanging fluids. Large energy losses are in turn associated with heat exchange irreversibilities or inefficiencies which waste the LNG refrigeration potential.
The prior art, in order to achieve reasonable process efficiencies under the above considerations, has deliberately kept process integrations of LNG vaporization and ethylene production simple in nature and limited in scope. Such restrictions minimize the number of refrigeration loads on the warming LNG and thereby permit a readily accomplished matching of LNG to system refrigeration requirements with moderate efficiency. The prior art has not, however, been able to efficiently employ LNG refrigeration in a complex ethylene facility having a front end depropanizer coupled with an extensive fore-cooling recovery section and final separation section. In prior art integrations, the fractionation of olefin-bearing feed gas has been carried out either at high (&gt;200 psig) pressures to provide reasonable matches between the supplied LNG and the system refrigeration requirements or else, when good matching is obtained, at low (&gt;200 psig) pressures, to take advantage of savings from reduced feed gas compression requirements. Neither of these approaches is completely desirable to the above-described ethylene plant having a front-end depropanizer and forecooling section. At high pressures, the overall investment and operating savings for the implementation of LNG refrigeration in this plant yield a modest economic advantage but the effectiveness of the supplied refrigeration is low. At low pressures, there exist imbalances in the refrigeration requirements over the various temperature ranges which would result in highly inefficient overall LNG utilization.
Accordingly, it is an object of the present invention to provide an improved process integration of ethylene production with LNG warming for regasification thereof.
It is a further object of the invention to implement LNG refrigeration in an ethylene plant of the type employing a front-end depropanizer and forecooling recovery section in a highly efficient manner.
It is a still further object of the invention to provide an implementation of LNG refrigeration in an ethylene plant which is characterized by higher levels of LNG refrigeration utilization and effectiveness than are achieved by the integration schemes of the prior art.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.